The failure plane in a direct shear test is the ______.
Try it first: pick an option, then read the explanation.
Correct answer: A. horizontal plane
Asked in SSC JE 15/11/2022 (Shift-I)
Detailed Explanation
A direct shear box is split horizontally into an upper and a lower half. During the test one half moves sideways relative to the other.
Why “horizontal plane” Is Correct
The specimen has no choice: it must shear along the horizontal plane at the joint between the two halves. This plane is fixed in advance and is not necessarily the weakest plane in the soil, which is a key limitation of the test.
| Feature | Direct shear test | Triaxial test |
|---|---|---|
| Failure plane | Forced: horizontal | Free to form on the weakest plane |
| Stress distribution | Non-uniform (higher at edges) | Uniform on the failure plane |
| Drainage control | None | Full control (UU, CU, CD) |
| Pore pressure measurement | Not possible | Possible |
| Stress state | Only on the failure plane | Known at every stage (σ₁, σ₃) |
| Best for | Sands, quick drained tests | All soils |
Why the Other Options Are Wrong
- B. major vertical plane: The shear plane is horizontal, not vertical.
- C. weakest major plane: The plane is predetermined by the box; it is not necessarily the weakest plane.
- D. principal vertical plane: Principal planes carry no shear stress, so failure cannot occur on a principal plane.
Key Point
Direct shear: failure plane forced and horizontal. Triaxial/UCS: failure plane free to develop.
Key Concepts for Students
- Plot τ against σ for several tests to get c and φ directly.
- Quick and simple; best suited to sands and drained tests.
- Ring shear tests extend the idea to very large displacements for residual strength.
Practice more: this is question 15 in Shear Strength of Soils MCQs – Part 1. Try the whole set and check every answer instantly.